I'm going to offer a different angle here, although probably not accurate, i'll say it anyway.
I've been looking at the latest images in Watchtower literature, and it seems that most of these "happy" and "joyfull" people are more and more ethnic-type (pardon the idiotic term) characters.
I wonder if this has anything to do with the fact that the fastest growing group of JWs in this country are the immigrant population. Corporations do this regularly when they "tune" their advertising to their target audience - let's say toyota always uses a young male, mid 20's, happy-go-lucky, with colorful clothes as the driver of their new sporty coupe in their ads.
Again, it's probably a stretch to offer this theory, but i'm really wondering if there is a strategy behind it.